The present invention relates to electric motors having a frictional planetary speed reducer incorporated therein.
In recent years, servomotors rotatable at a low speed of up to 10 r.p.m. are used as drive means for robots and like automatically controllable machines. Such servomotors include, for example, dc servomotors, ac servomotors, direct drive servomotors and stepping motors. Owing to their construction, however, these servomotors have the problem that the rotation in a low speed range involves marked irregularities or that the motor fails to give high torque.
To compensate for the above drawback of the servomotor in the low speed range, an electric motor has been proposed which has a speed reducer as connected in series with the forward end of its rotor shaft externally thereof (see Unexamined Japanese Patent Publication SHO 58-119746).
The proposed motor nevertheless has the problem that the speed reducer incorporated therein renders the motor corresponding greater in length and weight, consequently imposing limitations on its use.